The bag cell neurons of the marine opisthobranch mollusk Aplysia, synthesize and release, when active, a number of polypeptides and peptides. One of these peptides, pI 9.2, m.w. 4400, induces egg-laying and correlated behavior when injected into recipients. This neuropeptide, egg-laying hormone (ELH), has been purified and its primary structure determined. Pure ELH is known to have direct effects on the central nervous system of Aplysia. We have generated antibodies to ELH for a variety of biochemical and physiological experiments: with ELH antibody and the ELISA technique, we plan to quantitate levels of ELH in blood (during mating) and during in vitro discharge of the bag cells (BCs). With the immunoperoxidase techniques we have established, by immunocytochemistry, that ELH is synthesized in all BCs, investigated the detailed morphology and location of BC processes and determined that there are ELH-like cells in other parts of the CNS. We have also purified two reproductive trace peptides that activate the bag cells and thus cause egg laying. Their primary structures are now known. The BC neurons in each cluster discharge synchronously on synaptic activation in vitro and afterdischarge for approximately 30 minutes (at 14 degrees C) after synaptic activation. Following such an afterdischarge, the BC neurons become refractory, for generating another long afterdischarge, for several hours. We have been investigating the mechanisms by which the afterdischarge and subsequent refractoriness are generated using electrophysiological, pharmacological and biochemical techniques. Our present evidence indicates an increase in cAMP and protein phosphorylation during the early stage of afterdischarge. We are investigating the possibility that Ca 2 ion- and/or K ion-channels are phosphorylated leading to a long lasting depolarization which nediates afterdischarge.